No abstract

Vibrational ultrastrong coupling (USC), where the light-matter coupling strength is comparable to the vibrational frequency of molecules, presents new opportunities to probe the interactions of molecules with zero-point fluctuations, harness cavitymodified chemical reactions, and develop novel devices in the mid-infrared spectral range. Here we use epsilon-near-zero nanocavities filled with a model polar medium (SiO 2 ) to demonstrate USC between phonons and gap plasmons. We present classical and quantum mechanical models to quantitatively describe the observed plasmon-phonon USC phenomena and demonstrate a modal splitting of up to 50% of the resonant frequency (normalized coupling strength η > 0.25). Our wafer-scale nanocavity platform will enable a broad range of vibrational transitions to be harnessed for USC applications.Here we demonstrate vibrational USC within nanocavities, drastically reducing the size of the system and thus the amount of material involved in achieving MIR USC. Specifically, we use coaxial nanocavities, which exhibit strong transmission resonances and field enhancements close to the cutoff frequency of the TE 11 -like mode; these can be understood as resulting from excitation of the zeroth-order Fabry-Perot resonance 34 or, alternatively, as arising from an effective epsilon-near-zero (ENZ) effect [35][36][37] . We couple this nano-coax ENZ mode to the lattice vibrations of SiO 2 and demonstrate MIR USC with a mode splitting exceeding 50% of the vibrational transition frequency (η > 0.25) within a solid-state system.

We investigate transmission efficiency and finite size effects for the subwavelength hole arrays. Experiments and simulations show how the finite size effects depend strongly on the hole diameter. The transmission efficiency reaches an asymptotic upper value when the array is larger than the surface plasmon propagation length on the corrugated surface. By comparing the transmission of arrays with that of the corresponding single holes, the relative enhancement is found to increase as the hole diameter decreases. In the conditions of the experiments the enhancement is one to two orders of magnitude but there is no fundamental upper limit to this value.

The excitation of surface plasmon polaritons (SPP) by focusing a laser beam on single subwavelength holes opened in a thin gold film is studied both experimentally and theoretically. By means of leakage radiation microscopy, quantitative measurements of the light-SPP coupling efficiency are performed for holes with different sizes and shapes. The system is studied theoretically by using a modal expansion method to calculate the fraction of the incident energy which is scattered by the hole into a surface plasmon. We demonstrate that a single subwavelength hole can be used to generate SPP with an efficiency up to 28%.